An adapter includes a connector, a rectifier, a communication unit, a modulating unit, and a switching unit. The connector receives an input voltage. The rectifier is capable of rectifying the input voltage into various direct current voltages. The various direct current voltages include a predetermined voltage and a desired voltage. The communication unit is configured for generating a communication signal. The modulating unit is configured for superposing the communication signal to the predetermined direct current voltage to generate a modulation signal. The communication unit is further configured for receiving a desired signal comprising information of the desired voltage from the electronic device, and controlling the rectifier to rectify the input voltage into the desired voltage and controlling the switching unit to electrically connect the rectifier to the electronic device for transmitting the desired voltage to the electronic device when receiving the desired signal.